1. Field of the Invention
This invention belongs to a technical field of evacuation processes used in vacuum systems such as semiconductor device fabrication systems. More particularly, it belongs to a technical field of evacuation processes and systems for realizing high-speed soft evacuation, that can shorten evacuation time while controlling the movement of particles.
2. Related Background Art
Various vacuum systems are used in the fabrication of semiconductor devices, electric circuits and so forth. As thin-film forming systems, LPCVD (low-pressure chemical vapor deposition) systems are used to form Si.sub.3 N.sub.4 films for LOCOS masks and poly-Si (polycrystalline silicon) films for gate electrodes; plasma enhanced CVD systems, to form SiO.sub.2 films for interlayer insulation and SiN films for final protection; and sputtering systems, to form aluminum thin films for wiring. Besides, vacuum systems such as ion implantation systems, plasma etching systems and photoresist ashing systems are put into wide use.
FIG. 16 illustrates a block diagram of an evacuation system commonly used in load-lock type vacuum systems. Reference numeral 601 denotes a reactor in which the processing is carried out; 602, a high-vacuum pump such as a turbo-molecular pump, a cryopump, a diffusion pump or an adsorption pump; 603, a low-vacuum pump such as a dry pump or a rotary pump; 604, a roughing valve for evacuating the inside of the reactor 601 in the state of atmospheric pressure; 605, an evacuation conductance control valve; 606, a main-evacuation valve for performing main evacuation after the roughing; 607, a pump-to-pump valve provided between the high-vacuum pump 602 and the low-vacuum pump 603; 611, a load-lock chamber; 612, an evacuation pump for evacuating the inside of the load-lock chamber; 613, a gate valve for intercepting the passage between the reactor 601 and the load-lock chamber 611; and 614, an evacuation valve through which the inside of the load-lock chamber is evacuated.
The vacuum system as shown in FIG. 16 is operated in the following way to carry out evacuation and processing. First, keeping the valve 605 always open, the low-vacuum pump 603 is actuated to open the pump-to-pump valve 607, and thereafter the high-vacuum pump 602 is actuated. After the pump-to-pump valve 607 is closed, the roughing valve 604 is opened. Thus, the inside of the reactor 601 is started to be evacuated in the state of atmospheric pressure. After the reactor 601 has been evacuated until its internal pressure comes within the pressure range with which the high-vacuum pump 602 can operate for evacuation, the roughing valve 604 is closed and then the pump-to-pump valve 607 and the main-evacuation valve 606 are successively opened to initiate main evacuation. Usually, this state is kept except when the system is checked for maintenance. After a gate valve 613 is opened, a cassette holding a plurality of processing substrates is transported into the load-lock chamber 611. After the gate valve 615 is closed and the vacuum pump 612 is actuated, the evacuation valve 614 is opened. Thus, the inside of the load-lock chamber 611 is started to be evacuated in the state of atmospheric pressure until it has a reduced pressure. After the inside of the load-lock chamber 611 has been evacuated to a pressure low enough not to adversely affect the reactor 601, the gate valve 613 is opened to transport into the reactor 601 the substrates held in the cassette, and then the gate valve 613 is closed to carry out processing. During this processing, gases are introduced into the reactor from a processing gas feed system 616, and the pressure inside the reactor 601 is kept at a predetermined value by adjusting the conductance of the evacuation conductance control valve 605. After the processing is completed, the gate valve 613 is opened, and the substrate having been processed is put into the cassette left in the load-lock chamber 611 from the reactor 601, whereby the next substrate is transported into the reactor 601. After all the substrates have been processed, the evacuation valve 614 is closed to return the internal pressure of the load-lock chamber 611 to atmospheric pressure, and the cassette is taken out.
FIG. 17 illustrates a block diagram of an evacuation system commonly used in open-to-air type vacuum systems. Reference numeral 701 denotes a reactor in which the processing is carried out; 703, a vacuum pump; 704, a roughing valve for evacuating the inside of the reactor 701 in the state of atmospheric pressure; 705, an evacuation conductance control valve; 706, a main-evacuation valve for performing main evacuation after the roughing.
The vacuum system as shown in FIG. 17 is operated in the following way to carry out evacuation and processing. First, keeping the valve 705 always open, the evacuation pump 703 is actuated. After a gate valve 715 is opened, a processing substrate is transported into the reactor 701. The roughing valve 704 is opened. Thus, the inside of the reactor 701 is started to be evacuated in the state of atmospheric pressure. After the inside of the reactor 701 has been well evacuated, the roughing valve 704 is closed and then the main-evacuation valve 706 is opened to initiate main evacuation. During this processing, gases are introduced into the reactor from a processing gas feed system 716, and the pressure inside the reactor 701 is kept at a predetermined value by adjusting the conductance of the evacuation conductance control valve 705. After the processing is completed, the main-evacuation valve 706 is closed to return the internal pressure of the reactor 701 to atmospheric pressure, whereby the substrate is taken out, and the next substrate is put into the reactor.
In both of the load-lock type and open-to-air type vacuum systems, the evacuation conductance control valve 605 or 705 is set always to stand open (normally open) except when processing gases are introduced into the reactor, a vacuum vessel, and the pressure inside the vacuum vessel is maintained at a predetermined value, and the valve 605 or 705 is set to stand at a maximum conductance.
Accordingly, when the vacuum vessel is evacuated before the processing gases are introduced into the vacuum vessel, the evacuation is operated by only switching the open-close valves 604 and 606 or 704 and 706 to bring the reactor into the state of interception or the state of communication.
When processing objects (substrates) are processed in the vacuum vessel having been evacuated, using the evacuation system of the vacuum system shown in FIG. 16 or 17, it has not been easy to improve processing yield.
As a result of investigation on the cause, it became clear that particles adhering to processing objects in the vacuum system hindered the improvement in yield. As a countermeasure therefor, the inside of the vacuum vessel was cleaned, but the yield again began to decrease with progress in the processing. Accordingly, how the particles behaved was observed. As a result, it became clear that the particles, having stood still in the vacuum vessel when its inside was evacuated before the processing gases were introduced into the vacuum vessel, moved as if they flew up, and adhered to the processing objects.
The particles greatly fly up especially when the roughing is completed in a short time so as to shorten evacuation time and then the main evacuation is carried out. This is due to a great change in pressure at the time of the main evacuation.
If in order to control such movement of particles the main evacuation is started after the roughing is carried out for a long time to provide a sufficiently low pressure, it takes a long time for the evacuation in total, resulting in a low processing throughput for the processing objects. This greatly affects the vacuum system when it is the open-to-air type system the inside of the vacuum vessel of which is made open to atmosphere every time the processing object is put in and out, as utilized, e.g., in most ashing systems.